1. Field of the Invention
The present invention is directed to a process of manufacturing cones of drilling bits which have hard cutter inserts. More particularly, the present invention is directed to a process of laser hardening the outer shell and certain other surfaces of roller cone bits of the type which also have hard tungsten carbide or like cutter inserts.
2. Brief Description of the Prior Art
One important type of rotary drill bit used for subterranean drilling includes cutter cones which have hard tungsten carbide or like cutter inserts. Usually such cutter cones are rotatably mounted on journal legs of the drill bit so as to rotate as the drill bit is rotated. The drill bit may be rotated from the surface, or by a "downhole" motor. The tungsten carbide or like hard cutter inserts of cutter cones are pressed into insert holes formed in the external surface of the cutter cones. These tungsten carbide inserts bear against the rock formation at the bottom of the hole, crushing and chipping the rock as drilling proceeds.
Because rock drilling is a technically very demanding service, and because failure of a drilling bit can cause very costly interruption in the drilling process, the construction of rock bits must be very rugged. Usually the cones of the drilling bit are made of forged alloy steel, although powder metallurgy and related cones have also been described in the patent and technical literature. Bearing surfaces are located within the interior of the cones to enable rotatable mounting to the journal leg. An effective seal must be provided between the rotating cone and the journal leg so as to prevent escape of lubricating grease from the bearings, and to prevent entry of drilling fluid and other foreign matter in the bearing.
The steel body of the cone itself must be sufficiently ductile and tough so as to avoid fracture or shattering. Certain parts of the interior of the cone, particularly the ball bearing races, must be quite hard in order to provide sufficiently long bearing life. The exterior of the cutter cone ideally should also be quite hard and abrasion resistant so as to avoid rapid wear due to its exposure to the formation, and the highly abrasive and erosive action of the drilling fluid.
The tungsten carbide or other hard inserts in the roller cones must be held sufficiently strongly so as to prevent premature loss. The inserts must also be prevented from rotating in the insert holes, because rotation in the insert hole leads to decreased drilling efficiency and eventually to loss of the insert.
In view of the economic importance of subterranean drilling for oil and other minerals, the prior art has developed a variety of technological approaches to more or less satisfy the above-summarized requirements.
In accordance with one basic approach, the forged steel cone body is made of a "carburizable" low carbon steel, which, however, has sufficient ductility and toughness to be adequately resistant to fracture. Certain parts of the interior of the cone, such as the bearing races, may be carburized to increase their hardness, leaving the exterior of the cone without a hardened case.
Alternatively, the bearing races and the exterior shell of the cone may both be carburized. However, this alternative procedure has not been employed widely, because it is difficult to drill insert holes into the exterior shell through a hardened carburized case. Moreover, the obvious alternative of first drilling the insert holes, and thereafter carburizing the exterior shell, is also impractical because the interior of insert holes should not be carburized. This is because a hardened case in the insert holes would render the wall of the insert holes less ductile and less fracture resistant, and therefore would make press fitting of the hard inserts into the holes impractical or very difficult. Carburizing also tends to distort drilled holes.
In final analysis, carburizing rotary drilling bit cones is relatively labor consuming, because stop-off paint must be applied to the cone in several areas where hardening by carburization is not desired. Application of stop-off paint becomes particularly laborious, if carburization of the external shell is desired, because in this case the insert holes must be drilled first, and the stop-off paint must be applied to the insert holes as well. Moreover, little can be done to eliminate hole distortion from this high temperature heat treatment. In accordance with some prior art procedures, the exterior of the cone shell is carburized, but the carburized exterior case is removed in a finish machining operation before the insert holes are drilled.
In light of the foregoing difficulties, most roller cones have an exterior shell surface which is not carburized, and have a surface hardness of only approximately 42 Rockwell C (Rc) hardness units. Whereas the alloy steel of these cones is adequately ductile and tough, lack of external shell surface hardness and abrasion resistance results in relatively rapid wear and erosion of the cone shell during drilling, often resulting in loss of tungsten carbide inserts and inadequate bit performance.
Another alternative., described in U.S. Pat. No. 4,303,137, is to selectively heat treat and rapidly quench an interior surface layer of the ball bearing races of the roller cones, so as to form a hard martensitic layer and a hard bearing surface therein. This selective heat treatment may be accomplished by bombardment of the bearing races with a laser beam, as is described in U.S Pat. No. 4,303,137.
As is apparent from the foregoing, there is still a substantial need in the prior art for a process for substantially hardening, in an economically feasible manner, the exterior shell surface and other surfaces of hard insert bearing roller cones. The present invention provides such a process.